It is known to make various motor-vehicle parts—e.g. tie rods, B-columns, struts, door beams—of hardened steel with uniform ductility and grain structure throughout the entire workpiece. This is accomplished by a heat treatment of the part, raising it to a predetermined temperature and then quenching it in accordance with the desired characteristics of the finished product. The main factor affecting grain structure and/or ductility is the maximum temperature to which the workpiece is heated, that is whether or not it reaches any of several critical temperatures, the so-called AC1-AC4 points.
For specific parts, however, it is desirable for the grain structure to vary from one region to another. One region might need to have exceptional strength while another might need to be able to deform somewhat. This can be accomplished most simply by making the part as a composite of two pieces that are differently treated to have the desired characteristics.
U.S. Pat. No. 5,972,134 describes a one-piece part having regions of different ductility. It is produced by heating it locally before deforming it into the desired shape and then cooling it. The disadvantage of this method is that it is a complex batch operation that does not lend itself to the mass production needed for motor-vehicle manufacture.
German utility model 200 14 361 published 16 Nov. 2000 describes a door post or so-called B-column that is rendered austenitic in a furnace and then is simultaneously deformed and quenched in a die. Some parts of the workpiece are insulated before it is put in the furnace so that they do not become austenitic and thus do not when hardened develop a martensitic grain structure. Such a process is also unwieldy, involving the application and removal of insulation before and after the heat treatment, two extra steps that considerably elevate the cost of the workpiece.
For mass production of parts it is standard to use a continuous furnace through which the parts move along a path on a conveyor. U.S. Pat. No. 4,622,006 describes such a continuous-heating furnace which is provided with means for taking out and inserting workpieces at several locations along the treatment path. Thus it is possible, with a continuously operating furnace, to differently heat treat different workpieces, some staying in the furnace for the full treatment and others only being heated for substantially less time. While this system is indeed very flexible, it does not allow one to produce a single workpiece having regions that are treated differently.